The present invention relates to a head driving technique, for a liquid ejecting apparatus, that detects an increase to a predetermined temperature at a switching circuit IC, which sequentially switches and drives pressure generating elements provided for corresponding nozzles through which liquid droplets are ejected, and that halts a liquid ejecting operation.
The liquid ejecting device is used as a record apparatus used with an image record apparatus, a color material ejecting apparatus used for manufacturing a color filter of a liquid crystal display, etc., an electrode material (conductive paste) ejecting apparatus used for electrode formation of an organic EL display, an FED (face light emitting display), etc., a bioorganic substance ejecting apparatus used for biochip manufacturing, a specimen ejecting apparatus as a precision pipet, etc. One form of the liquid ejecting devices will be discussed by taking an ink jet printer as an example.
Ink jet color printers, used for the ejection from recording heads of several colors of ink, have become popular as output apparatuses for computers, and have been employed for the printing, using multiple colors and tones, of images processed by the computers.
For example, in an ink jet printer using a plurality of piezoelectric elements as driving elements, the piezoelectric elements corresponding a plurality of nozzles of a print head, are selectively driven, and ink droplets are ejected from the nozzles in accordance with the drive voltages applied to the individual piezoelectric elements, thereby the ink droplets are deposited as dots on a printing sheet for printing.
The piezoelectric elements are corresponded to the nozzles for ejecting ink droplets. The ink droplets are ejected based on drive signals supplied by at least one head driver IC mounted in the print head.
This type of head driving device is shown in FIG. 6. In FIG. 6, a head driving device 1 includes piezoelectric elements 2, a drive waveform generating circuit 3, current amplifier circuits 4 and switch circuits 5. Each of the piezoelectric elements 2 is provided so as to correspond with each of a plurality of nozzles of an ink jet printer. The drive waveform generating circuit 3 supplies a drive signal to an electrode 2a of each of the individual piezoelectric elements 2. One each of the current amplifier circuits 4 and the switch circuits 5 is located between the drive waveform generating circuit 3 and each piezoelectric element 2.
While only one piezoelectric element 2 is shown in FIG. 6, since a plurality of nozzles are provided in the head of an ink jet printer, a plurality of piezoelectric elements are supplied, one for each of the nozzles. A drive signal COM, generated by the drive waveform generating circuit 3, is sequentially output, through a shift register, to each of the piezoelectric elements 2.
The piezoelectric elements 2 can be displaced by voltages applied to electrodes 2a and 2b. Further, a charge, at a level near the intermediate potential, is constantly applied to the piezoelectric elements 2. When a discharge is initiated based on the drive signal COM supplied by the drive waveform generating circuit 3, ink droplets are ejected by applying pressure on the ink supplied for corresponding nozzles.
The drive waveform generating circuit 3 generates the drive signal COM that is transmitted to the head of the ink jet printer. The drive waveform generating circuit 3 may be located in either the printer main body or the printing head.
The current amplifier circuit 4 includes two drive devices, i.e., first and second transistors 4 and 4b. For the first transistor 4a, the collector is connected to a constant voltage power source, the base of which is connected to a first output terminal of the drive waveform generating circuit 3, and the emitter of which is connected to the input terminal of the switch circuit 5. With this arrangement, upon the reception of the drive signal COM from the drive waveform generating circuit 3, the first transistor 4a is rendered active and transmits a charge current through the switching circuit 5 to the piezoelectric element 2.
For the second transistor 4b, the emitter is connected to the input terminal of the switching circuit 5, the base of which is connected to a second output terminal of the drive waveform generating circuit 3, and the collector of which is grounded. With this arrangement, upon the reception of a drive signal COM from the drive waveform generating circuit 3, the second transistor 4b discharges the piezoelectric element 2 through the switching circuit 5.
Based on a control signal, the switching circuit 5 is turned on at the timing whereat a corresponding piezoelectric element 2 is driven, and outputs the drive signal COM to this piezoelectric element 2. The switching circuit 5 is actually a so-called transmission gate that turns a corresponding piezoelectric element 2 on or off, and is integrated to serve as the switching circuit IC 6.
For the thus arranged head driving device 1, the switching circuit ICs 6, constituting the switching circuits 5, generate heat as they are activated, and this heat is discharged by the ejection of ink droplets from the piezoelectric elements 2, or through the part that constitutes the head. However, due to the continuous driving operation, or the exhaustion of ink, the satisfactory discharge of heat through ink ejection will not be performed.
When in this state printing is continued, the temperature of each switching circuit IC 6 is increased, and thermal destruction of the switching circuit IC 6 and the piezoelectric element may occur. Therefore, for the related ink jet printer 1, based on the fact that the anode voltage of a diode 7, which is provided for each switching circuit IC 6, is changed in accordance with a temperature of each switching circuit IC 6, the anode voltages of the diodes 7a, 7b, 7c and 7d in the switching circuit ICs 6a, 6b, 6c and 6d are transmitted through cables 8a, 8b, 8c and 8d to a controller 9 that is arranged in the printer as an ASIC.
In the controller 9, the anode voltages of the switching circuit ICs 6a, 6b, 6c and 6d are converted into digital values by an AD converter 9a to detect the anode voltages of the diodes 7a, 7b, 7c and 7d of the switching circuit ICs 6a, 6b, 6c and 6d. The temperatures of the switching circuit ICs 6a, 6b, 6c and 6d are detected based on the anode voltages.
When a predetermined temperature or higher is detected for the switching circuit IC 6a, 6b, 6c or 6d, the controller 9 temporarily halts the printing operation to reduce the temperature of the pertinent switching circuit IC 6. Also, when the ink is exhausted, the controller 9 halts the printing operation until an ink cartridge exchange is performed.
However, since the controller 9 performs AD conversions for the anode voltages of the switching circuit ICs 6a, 6b, 6c and 6d, a long processing time is required to detect the temperatures of the switching circuit ICs 6a, 6b, 6c and 6d. Accordingly, there is a comparative reduction in the accuracy of the temperature measurement, and until the next temperature measurement can be made, a large, estimated value must be employed as a temperature rise. Therefore, the ON resistances of the analog switches for the switching circuits 5 in the switching circuit ICs 6a, 6b, 6c and 6d must be reduced.
When the ON resistance of each analog switch is small, the sizes of the switching circuit ICs 6a, 6b, 6c and 6d are increased, and the manufacturing costs are also raised.
Further, since comparatively long connection cables extend from the switching circuit ICs 6a, 6b, 6c and 6d to the controller 9, and since analog signals transmitted through these cables tend to be adversely affected by noise, the detection accuracy is reduced.
Furthermore, the AD converter 9a provided on the controller 9 is required, since the AD converter 9a converts the anode voltages of the switching circuit ICs 6a, 6b, 6c and 6d into digital signals. Therefore, the size of the controller 9 constituted by an ASIC is increased.